The invention relates to monitoring the emissions of a supply vessel for storing a volatile medium including a fuel tank system mounted in a motor vehicle. The invention relates especially to a method, a circuit as well as a control apparatus for monitoring the emissions of such a supply vessel during operation.
In various areas of technology, supply vessels of the above-mentioned kind have to be checked with respect to their tightness. Accordingly, it is important, for example, in chemical processing technology to check the tightness of tanks for keeping volatile chemical substances for reasons of emission protection. The necessity is especially present in the area of motor vehicle technology to regularly carry out tightness checks on fuel tanks or fuel tank systems utilized in motor vehicles.
In the last-mentioned context, reference is made to the statutory regulations present in parts of the United States for the operation of internal combustion engines. According to these regulations, it is necessary that motor vehicles, which utilize volatile fuels such as gasoline, have a device for monitoring the emission of fuel which can detect a non-tightness or leakage of the size of 0.5 mm in the tank system utilizing only on-board equipment.
U.S. Pat. No. 6,234,152 discloses a method for checking the tightness of a vehicle tank system. Here, an overpressure relative to ambient pressure is introduced into the tank system and a conclusion as to a leakage is made from the subsequent trace of the pressure. Similar methods for checking a tank-venting system of a motor vehicle are presented in U.S. Pat. Nos. 5,890,474 and 6,131,550.
In the above-mentioned monitoring of emissions and especially the detection of the smallest leakages of the above-mentioned cross section of 0.5 mm, the invention is based on the recognition that the temperature of the volatile medium has a considerable influence on the measuring accuracy in a tightness check (leakage diagnosis). On the one hand, the above-mentioned operational checks, especially in tank systems, should be carried out only within a specific temperature range because, with increasing fuel temperature, the vaporization of the medium increases and, starting at a specific temperature, an overpressure develops in the supply vessel because of vaporization and this overpressure increases the overpressure generated in the tightness check or counters the generated underpressure. In this way, incorrect assumptions with respect to the pressure conditions define a reason for incorrect diagnoses. Accordingly, in the case of a diagnosis carried out with overpressure, an un-tight supply vessel is diagnosed as xe2x80x9ctightxe2x80x9d and in a diagnosis carried out with an underpressure, a vessel which is indeed tight is erroneously diagnosed as xe2x80x9cun-tightxe2x80x9d.
In addition, especially for vessels manufactured of plastic, the thermal expansion of the material is to be considered. Based on the expansion characteristic of the plastic, which occurs with increasing temperature, uncontrollable volume changes of the interior vessel space occur and therefore, in turn, incorrect assumptions with respect to the existing internal pressure conditions result.
It is further noted that the term xe2x80x9csupply vesselxe2x80x9d (for example, in the case of motor vehicle tank systems) includes also function elements, which are significant for the entire tank system, such as lines and seals.
In view of the foregoing, it is an object of the invention to provide a method, a circuit and a control apparatus of the kind set forth initially herein which make possible a monitoring of emissions in supply vessels which is improved compared to the state of the art. Especially, this improvement should be achieved by detecting the actual temperature of the supplied medium with the least possible technical complexity especially by avoiding the use of costly temperature sensors in the supply vessel in order to increase the accuracy of a tightness check carried out on the supply vessel.
The method of the invention is for monitoring the emissions of a supply vessel storing a volatile medium during operation including a fuel supply tank of a motor vehicle. The method includes the steps of: carrying out a tightness check of the supply vessel from time-to-time; determining the temperature of the medium based on at least one characteristic variable utilizing a model computation from time-to-time or cyclically; and, either utilizing the temperature in the tightness check or carrying out the tightness check only when the determined temperature of the medium lies within a pregivable temperature interval.
The invention is based on the idea to include the temperature of the volatile medium in a function check described initially herein as a corrective quantity and model this corrective quantity based on additional characteristic variables, that is, determine this corrective quantity based on a model computation. The additional characteristic variables include variables such as the ambient temperature, the fill level of the supply vessel or, in the case of a motor vehicle, additionally, operating data of the vehicle (vehicle speed or the like) or of the vehicle engine (duration of operation, the length of time an engine has been switched off, engine temperature or the like).
According to a first variation, the invention provides to mathematically determine the real temperature of the medium (T_ktm) from these characteristic variables and to include the value of T_ktm which is so computed as a corrective quantity in the check of the operability of the supply vessel as mentioned above. In a second variation, a check as to operability of the supply vessel is only carried out when the computed value of T_ktm lies within a pregivable temperature interval.
In the above-mentioned variations, the corrective quantity can be determined by means of the model computation before each execution of an operational check or from time-to-time, for example, cyclically. Alternatively, the characteristic variables, which are necessary for the computation of T_ktm, can be stored after a one-time executed model computation in the form of a characteristic variable diagram or in a corresponding table especially for a given construction type of the supply vessel or of a motor vehicle. In this way, the characteristic variables are directly available for subsequent determinations of T_ktm without it being necessary to carry out the above-mentioned model computation anew.
For further refining the suggested method, characteristic variables can be included in the model computation and these characteristic variables include the operation duration or switch-off duration of an internal combustion engine, which is supplied by the supply vessel, as well as, in the case of a vehicle, the road speed, the fuel level in dependence upon the vehicle speed and/or the elevation of the supply vessel or a vehicle having such a vessel. For falling ambient temperatures at a simultaneously relatively high geographic elevation of the vehicle (for example, during travel through mountain passes), one can assume a reduced rate of cooling because of the reduced air pressure. In addition, for vehicles, the characteristic data concerning the particular vehicle manufacturing series can be included such as the type of chassis and/or type of engine. In this way, the following can advantageously form a basis: different flow conditions for a moving vehicle and a different underflow of a vehicle tank caused thereby as well as different mounting positions of the fuel tank and/or of the engine in the vehicle chassis in dependence upon the chassis form. When a shut-down duration of the engine is included in the model computation, it can be provided to store a cool-down curve specific to the model series and to apply this curve as a starting value for the engine temperature when the engine is started again.
It is noted that the warming curve and/or the cool-down curve of the medium in the supply vessel, which is to be considered in the context of the model computation, in a motor vehicle and in other uses of the supplied vessel, are dependent from the present fill level as well as on the particular manufacturing series of the vessel. Accordingly, a relatively high fill level leads to a slower warming of the stored medium because of the correspondingly high thermal capacity of the medium and a relatively low fill level leads to a more rapid warming. In the above-mentioned model computation, these interrelations are considered in accordance with a further embodiment.
The ambient temperature can be considered in the determination of T_ktm (for example, multiplicatively) because the particular ambient temperature, the warming curve and the cool-down curve of the medium have considerable influence. In the case of a fuel vessel mounted in a motor vehicle, the following can be considered or be included as corrective variables in the warming curve and/or cool-down curve: vehicle operating variables and/or engine operating variables (such as the instantaneous or average engine load), vehicle road speed and/or the selected transmission gear.
In a further embodiment, T_ktm is only determined from one or several characteristic variables when the above-mentioned characteristic variables lie within a pregivable variant range, that is, when the particular characteristic variable performs sufficiently constant over a pregivable time interval. Alternatively or in addition, it can be provided that a new determination of T_ktm takes place only when the vehicle speed and/or the duration of operation of the engine exceeds a pregivable limit value. In this way, it is ensured that the influence of fluctuations of the detected characteristic variables, which are caused by a situation or the environment, is minimized to the value of T_ktm which was computed from these characteristic values. In this way, it is ensured that the engine has reached the operating temperature and that a subsequent warming of the engine leads to no further increase of T_ktm. The waiting time can be fixed in dependence upon engine type and/or chassis form of the vehicle, for example, separately for individual vehicle series.
To further increase the reliability of the function check, it can be provided that a T_ktm (which is determined during operation of an engine connected to the supply vessel or determined in the operation of a vehicle having such a vessel) is intermediately stored and is compared to an instantaneous ambient temperature which is measured for a subsequent starting of the engine or of the vehicle. Until the subsequent new determination of T_ktm, the particular greater of the two values is applied as the start value for T_ktm. With this maximum selection, an external heating of the stored medium is considered during a shutoff time of the vehicle, for example, based on a warming of the chassis and/or of the tank caused by solar radiation. Likewise, the influence of the specific geographical position in elevation of the vehicle can be considered.
In accordance with a further configuration, T_ktm is also determined during travel of the vehicle in order to consider the influence of the thermal backup on the fuel temperature. This thermal backup occurs during operation of the engine in the vicinity of the tank.
In addition, the thermal inputs of an electric fuel pump, an engine exhaust-gas system and/or a climate control, which cools the interior of the vehicle, can be considered.
T_ktm can also change after a tanking operation with a medium having a deviating temperature. For this reason, and in accordance with a further embodiment, changes of the tank fill level after a tanking operation are detected, which tanking operation is detected in a manner known per se, for example, by means of a tank cap sensor. As mentioned above, the adjustment of a temperature equilibrium can be awaited until a new determination of T_ktm takes place. Until the new determination, an approximation value (for example, the mean value from the last-stored value of T_ktm and the current ambient temperature) can form the basis which affords the advantage that an adequate value is present at least until that time. Furthermore, a tanking operation, which takes place during an interruption of the operation of the vehicle, can be detected in that, after the start of the engine, the difference between the current tank fill level and the intermediately-stored tank fill level value exceeds a pregivable threshold value. It is noted that the quantity of the after-tanked medium can be included in the model computation when T_ktm is newly computed after a tanking operation.
In the result, the invention makes possible the use of cost-effective plastic tanks, for example, in combustion-driven motor vehicles without an expensive temperature sensor mounted in the supply vessel and required for a leakage diagnosis of the size of 0.5 mm. For motor vehicles driven with flexible fuel (that is, in hybrid operation, ethanol/methanol), the invention makes possible the detection of critical vapor temperatures.
Furthermore, when there is a fault of one or several sensors (temperature sensor, tank fill level sensor, et cetera), nonetheless the determination of a useable T_ktm is made possible from the data being available. If, for example, the malfunction of a temperature sensor is recognized in the manner known per se, a substitute value, which is determined empirically in the model equation (for example, a mean value of 20xc2x0 C.) can be assigned to the corresponding temperature variable. Correspondingly, for a defective fill level sensor, a last stored value of T_ktm can be applied in lieu of an actually determined T_ktm value.
In order to suppress still more effectively incorrect T_ktm values in the mentioned sensor malfunction or to avoid such falsifications for intensely changing ambient conditions, a plausibility check can be additionally carried out wherein an actually determined T_ktm is compared to pregivable upper and/or lower limit values and is only assumed to be correct when T_ktm lies within these limit values. Additionally, and when a limit value is exceeded, the actual value can be assumed equal to the limit value.
The invention can be advantageously realized in an existing control apparatus, for example, an engine control apparatus, in the form of a control program. Here, it is advantageous that some or all of the above-mentioned characteristic values are already detected and present in such a control apparatus. Alternatively, the invention can be realized in the form of a circuit, for example, as an application specific integrated circuit (ASIC). The basis-forming model computation can be realized in the form of a binary-logic circuit loop formed of several stages. Each stage is viewed as a filter for the influence of the particular characteristic value on T_ktm. The attenuation of the particular filter is dependent upon the characteristic variables and the corrective variables which are dependent upon the ambient. Preferably, at least two filters form the basis in the model computation. Accordingly, in a first filter, the ambient air temperature as well as the position in elevation of the vehicle are included and, in the second filter, the tank fill level, the vehicle shutoff time and/or the engine shutoff time and the duration of operation are included.
In one embodiment, the control apparatus included in the invention or the circuit includes a read/write memory (RAM) for storing the above-mentioned characteristic variable diagrams and/or for intermediately storing a T_ktm value which is already determined. The read/write memory serves the above-mentioned purpose.
The invention is basically applicable in supply vessels in all areas of technology wherein volatile substances are stored in such vessels. In addition, it is understood that the term xe2x80x9cstorage vesselxe2x80x9d also includes tank systems or the like including their additional components.
The value for T_ktm, which is determined in accordance with the invention, can also be used as a corrective quantity in similar functions such as the above-mentioned function check, for example, for a tank-venting function mentioned initially herein.